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--- tutorials:bash_scripting:part4 [2012/06/19 01:46] – rmiles
+++ tutorials:bash_scripting:part4 [2017/10/12 10:58] (current) – external edit 127.0.0.1
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 //**Building a script with functions that encodes and shrinks videos**//
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-I can use a short one command script to save me the trouble of remembering (and typing) a very long //mencoder// command that encodes the video to a good DVD quality mpg-ps but the problem is that often I end up with a video that is too big to fit on a DVD and then I have to shrink the video before I can burn it. Below is an example of a movie before and after encoding.
+I can use a a short one command script to save me the trouble of remembering (and typing) a very long //mencoder// command that encodes the video to a good DVD quality mpg-ps but the problem is that often I end up with a video that is too big to fit on a DVD and then I have to shrink the video before I can burn it. Below is an example of a movie before and after encoding.
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 In this script the video is assigned to the variable //input_file// and then //input_file// is encoded with mencoder. If no video file was entered the user is given the correct usage of the script and the script exits.
-I use mencoder (from MPlayer) for encoding my videos. The command options in the script are provided in the MPlayer manual //**7.8. Using MEncoder to create VCD/SVCD/DVD-compliant files**// http://www.mplayerhq.hu/DOCS/HTML/en/menc-feat-vcd-dvd.html and work fine with anything I have saved on my two DVB-T cards from ABC and SBS. Note that MPlayer has been forked. There is now an MPlayer2 which does not include mencoder. The mencoder command can be substituded with an ffmpeg command.
+I use mencoder (from MPlayer)for encoding my videos. The command options in the script are provided in the MPlayer manual //**7.8. Using MEncoder to create VCD/SVCD/DVD-compliant files**// http://www.mplayerhq.hu/DOCS/HTML/en/menc-feat-vcd-dvd.html and work fine with anything I have saved on my two DVB-T cards from ABC and SBS. Note that MPlayer has been forked. There is now an MPlayer2 which does not include mencoder. The mencoder command can be substituded with an ffmpeg command.
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-The linux operating systems use a block size of 1024 bytes, i.e.1kb. I want the script to shrink the video if the video exceeds 4.2gb and 4,400,000kb equals 4.19gb, close enough for me.
+The linux operating systems use a block size of 1024 bytes, i.e.1kb. I want the script to shrink the video if the video exceeds 4.2gb and 4,400,000kb equals 1.19gb, close enough for me.
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-Bash will do simple math and it only works with integers. For floating point math I have to use //bc//. The shrink factor (a.k.a. requant factor) is found by dividing the file size by my set limit of 4,400,000kb which will result in a decimal. In this usage the equation is piped to //bc// using an //echo// command. The parameter //scale=X// can also be passed to bc to limit the number of decimal places printed to stdout.
+Bash will do simple math but for floating point math I have to use //bc//. The requant factor (read shrink factor) is found by dividing the file size by my set limit of 4,400,000kb which will result in a decimal. In this usage the equation is piped to //bc// using an //echo// command. The parameter //scale=X// cab be passed to bc to limit the number of decimal places printed to stdout.
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-My //con2vob// script listed above is too short to require functions but it will serve as a good example of how to use functions. It will also serve as the core of a more complex script that also burns DVDs which I will introduce in Part 5 of this series.
+My //con2vob// script listed above is too short to require functions but it will serve as a good example of how to use functions.
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 </code>
-In this script the two variable declarations are listed first. The variable //input_file// must be declared before the three functions because the variable is used in all three functions. The variable //dvd_size// does not have to be declared until required in the function //shrink_video// but I like to list all all variables I may want to shange at the beginning of the script where they are easy to find if I want to change them.
+In this script two variable declarations are listed first. The variable //input_file// must be declared before the three functions because that variable is used in all three functions. The variable //dvd_size// does not have to be declared until required in the function //shrink_video// but I like to list all all variables I may want to change at the beginning of the script where they are easy to find if I want to change them.
-Note that I have changed the notation when I use stdout from a command to assign a value to a variable. The use of backticks " variable=`some command`  " for command substitution has been depreciated and the new way is to use " $()" like this " variable=$(some command) " Backticks can still be used but you may have mixed results and they can not be used when using nested commands.
+Note that I have changed the notation when I use stdout from a command to assign a value to a variable. The use of backticks, e.g. //variable=`some command`// for command substitution has been depreciated and the new way is to use " $() " like this  //variable=$(some command)// Backticks can still be used but you may have mixed results and they should not be used when using nested commands.
-It is possible to do math in Bash if the numbers involved are all integers and if you use math for command substitution you can not use double quotes and you must use double quotes.
+It is possible to do math in Bash if the numbers involved are all integers. In such instances you must use double quotes.
 <code>bash-4.1$ result=`8-2` ;  echo "$result"
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 </code>
-**in progess**
+In //new-con2vob// I have three distinct operations which are easily tucked away into three functions. They are called one after the other by listing their names after they have been declared. In bash, functions can only be called after they have been declared.
+In **Part 5** of this series I will present a script that encodes shrinks and burns to a DVD. It is a little more complicated so it is a good example to show how handy functions can be in both planning and writing a script.
+----
+//There are gui programs out there which will do exactly what my con2vob script does but how many mouse clicks, how much time and how much memory does it take? I started scripted encoding and burning when I had a server for printer and file shares that also had a dvb card on board so saved movies and docos would be availble to any client on demand. Encoding takes a long time and is resource intensive but it didn't matter to a headless box with memory to spare and it could be done any time, day or night. It was just a matter of ssh'ing into the server and running a script.//
+I//'m not running that server now but I still prefer to do this sort of thing with scripts. I only use it on saved DVB broadcasts so the settings can remain the same. I either run con2vob with //nice// and go about doing something else or else run it when I go to bed or walk out the door and let the machine shut itself down when it is finished.//
+----
+**Cheers!**